I have Neons with my betta and they are perfectly fine. I love them in my tank. When you go buy them get the more mellow ones though. Just make sure your tank is big enough. Nothing less than 10 gallons if your adding other fish

It will all depends on your Betta,
Some will get along with most anything, While others will be more picky,
And then others will tolerate nothing else.
It is always best to have a separate tank prepared in case you have such a betta.

The Betta should show interest in any fish you add to his tank and will often flare up while he inspects it,
If he accepts it he should back off shortly and not bother it again, If he starts to "Dive" into it then it isn't going to work, If you got a really good betta he may not even care to check it out, The flexibility with betta's really is hard to say until you try it..

Adding the betta last sometimes helps and sometimes it leads to a false assumption that he's going to get along to only harm or kill it when your not expecting it.